There are a number of commercial processes that utilize a liquid phase, ebullating bed reactor. Some examples of these include H-Oil (heavy oil) and LC-Fining units (Lummus Citgo process). These reactors consist of a bubbling liquid or medium that causes the solid catalyst therein to remain fluidized. To enhance this fluidized state, a gas reactant is introduced into the liquid at the bottom of the reactor and leaves at the top along with any liquid throughput or carryover. When this bubbling liquid within the reactor reaches a certain height, it is separated out from the gas by spilling over into a recycle cup. This removed liquid then flows through a downcomer tube which returns the collected liquid to ebullating pumps for subsequent recirculation within the reactor. Any vapor product from the reactor, along with any unconverted liquid, exits the reactor and flows to the downstream recovery section of the process.
A major cause of down-time in ebullating bed reactors is due to an interruption in liquid or gas flow within the reactor. When the liquid level in the reactor falls below the top opening of the downcomer, no liquid is returned to the ebullating pumps thereby causing them to stop operating. Any number of process conditions can cause such a loss of liquid flow or such a reduction in liquid level. Some of these conditions are quite minor, such as changes in composition of feed or a partial loss of flow from the recycle compressor. As a result, the reactor is usually only on-stream about 80% to 90% of the time. When ebullation or fluidization is lost, the catalyst slumps and although feed may continue if this condition is not detected, heat is not removed to any significant degree. Thus, the unit must be shut down for a considerable period of time before ebullation can be re-established. In addition, past cases of lost circulation have resulted in catastrophic failure of ebullating units.
In U.S. Pat. No. 3,523,763 to Van Driesen et al., there is disclosed a catalytic reactor having a central recycle conduit designed for gas-liquid separation. The upper region of this conduit contains numerous, equally spaced openings which are partially blocked so as to restrict the amount and kind of gas-liquid separation that can occur. Such an upper location of these openings is to enhance the gas-liquid separation occurring in that region of the reactor. While this patent is intended to improve normal operation and further gas-liquid separation, it is not designed to reduce or prevent upset conditions from causing the reactor to shut down nor is it designed for liquid-solid separation nor is it designed for such separation along the length of the conduit as is currently needed.
It is thus an object of this invention to provide an ebullating bed reactor that is configured so that such down time is minimized. A further object of this invention is to provide a downcomer that can recirculate fluid back to the ebullating pumps even should the liquid level fall well below the elevation of its top opening. Still another object of this invention is to provide a downcomer that enhances liquid-solid separation both while the liquid is at its normal operating level and when the liquid is below such level. Yet another object of this invention is to eliminate or reduce the occurrence of upset conditions within the reactor which would cause the reactor to shut down. These and other objects and intentions will be obvious upon further investigation.